Image scanners convert a visible image on a document or photograph, or an image in a transparent medium, into an electronic form suitable for copying, storing or processing by a computer. An image scanner may be a separate device or an image scanner may be a part of a copier, part of a facsimile machine, or part of a multipurpose device. Reflective image scanners typically have a controlled source of light, and light is reflected off the surface of a document, through an optics system, and onto an array of photosensitive devices. The photosensitive devices convert received light intensity into an electronic signal. Transparency image scanners pass light through a transparent image, for example a photographic positive slide, through an optics system, and then onto an array of photosensitive devices.
Each photosensitive element, in conjunction with the scanner optics system, measures light intensity from an effective area on the document defining a picture element (pixel) on the image being scanned. Optical sampling rate is often expressed as pixels per inch (or mm) as measured on the document (or object, or transparency) being scanned.
Common photosensor technologies include Charge Coupled Devices (CCD), Charge Injection Devices (CID), Complementary-Metal-Oxide (CMOS) devices, and solar cells. Typically, for a CID or a CMOS array, each photosensitive element is addressable. In contrast, CCD linear arrays commonly serially transfer all the charges, bucket-brigade style, from each line array of photosensitive elements to a small number of sense nodes for conversion of charge into a measurable voltage. The present patent document is primarily concerned with photosensor arrays having serial charge transfer registers, also called serial readout registers.
In general, there is an ongoing demand for increased resolution and speed, improved color quality and image quality, and reduced cost. These demands often directly conflict and require trade-offs. Areas of an image with slowly varying color, particularly dark colors, need high bit depth and high signal-to-noise to accurately reproduce the smooth tone and texture of the original, but high sampling rate is not needed because there is no high frequency information in the image. Areas of an image that change color rapidly, for example a forest scene, or a close-up photograph of a multi-colored fabric, need a high sampling rate to capture the high frequency information, but high bit depth and high signal-to-noise are not needed. That is, for high frequency information, the color accuracy of each individual pixel is less important. Sensors having a relatively small area (with a correspondingly small pitch) provide high sampling rates, but typically have relatively low signal-to-noise ratios, relatively low bit depth, and relatively low scanning speed. Large sensor areas provide high signal-to-noise, high bit depth, and high speed, but cannot provide high sampling rates.
U.S. patent application Ser. No. 09/430,471 discloses photosensor assemblies that have multiple rows of sensors, with some rows having relatively small sensor areas and other rows having relatively large sensor areas. Different rows may be used for different scans, depending on the needs of each scan. U.S. patent application Ser. Nos. 09/703,960 and 10/080,217 disclose photosensor assemblies with multiple rows of sensors of different sizes, with various structures shared by multiple rows, thereby reducing size and cost. There is a need for additional improvement in photosensor arrays.